one more dumb question for you guys...so while draining and refilling either with water or coolant depending on which step your on...should the engine be off or running while draining and refilling. I had someone tell me that you should always keep an engine running while draining and refilling cause cracked blocks occur if engine is off? So engine off or on while draining and refilling?

 

The engine shouldn't be hot enough to cause any harm when either filling or draining. So, off when draining. Also off when filling as much as it will take, then start it and it will take some more as the bubbles rise.

I think what your referring to is an engine that has overheated and needs coolant added. I think the preference would still be to let it cool before adding cold coolant, but if you had to add to a hot engine it is better if it is pumping. That's a situation you want to avoid, and really has nothing to do with a routine flush and fill.

 

when you put the coolant in, mix it before hand.